Posted
by
Zonk
on Sunday February 13, 2005 @03:27PM
from the you-changed-the-outcome-by-measuring-it! dept.

ThePolkapunk writes "Scientists have been having problems in predicting the behavior of Quantum Dots, which are considered to be the most likely material to be used to build nanocomputers. Physorg is reporting that physicists at Ohio University believe they've found the problem, and it's with a flaw in the construction of quantum dots. If their theory pans out, "It's one more step towards the holy grail of finding a better quantum bit, which hopefully will lead to a quantum computer."" We first mentioned this about six years ago.

We have The Ohio State University and Ohio University, but no University of Ohio... at least not in Ohio. [NOTE: There are a number of other state-funded Universities: Bowling Green, Toledo, Akron, Kent, Miami, Case Western Reserve, et. al., but none of them have 'Ohio' in their name, except maybe Miami, which is often called "Miami of Ohio" to distinguish it from Miami University in Florida.]

This only applies to self-assembled quantum dots. The ones created by lithography or otherwise manually constructed didn't have this problem in the first place. Don't get too excited (unless you're working with photoexcitation in self-assembled QDs, in which case this might matter to you).

How do you verify the accuracy of Slashdot stories?
We don't. You do.:) If something seems outrageous, we might look for some corroboration, but as a rule, we regard this as the responsibility of the submitter and the audience. This is why it's important to read comments. You might find something that refutes, or supports, the story in the main.
Answered by: CmdrTaco
Last Modified: 10/28/00

Is there some relationship between/. and PhysOrg? If so, Commander Taco should be ashamed of it... PhysOrg is an eyeball tarpit, it NEVER credits the original article or provides a link back to it. Never. Not once. It might as well be dead trees...

This seems to be as good a place as any to try to clear up some misunderstandings. The company where I work is trying to hire a bunch of experts in Quantum Dots, and I've seen probably a dozen presentations from different researchers applying for jobs in the past six months.

First, I know the terms Q-dots is a trademark, and I think "Quantum Dots" might be trademarked by the same company. So don't give them so much mindshare, since that company isn't really even on the "forefront" of the technology. Call them fluorescent semiconductor nanocrystals, because it actually describes what they are, so people won't think they're being used in quantum computing (not yet, at least).

Second, these nanocrystals blink. Every researcher I've seen speak about these things mentions the blinking, but only recently did I hear someone give an explanation: poor surface coating allows electrons to leak out of the the crystal.

Third, Semiconductor nanocrystals are made of several layers. The central layer is usually Cadmium Selenide (CdSe), coated by Zinc Selenide. The second coating has a higher band gap energy, so electrons get "stuck" inside the nanocrystal and then emit photons when they drop back to the ground state. Unfortunately, these nanocrystals are very sticky without more coatings. Often a PEG (polyethylene glycol) linker is stuck on the outside of the ZnSe surface to inhibit these non-specific binding events.

Last, semiconductor nanocrystals are cool because you can excite them at many wavelengths, but the emitted photon's wavelength (color) depends on the size of the crystal being illuminated. The bigger the crystal, the redder the emission. That makes them size tunable, and easily multiplexible. Eventually, that could be really useful for quantum computing (or digital video, possibly).

And with RSA & friends no longer usable, how do you propose to distribute your AES etc. keys? It doesn't matter how strong your symetric cipher is, if you can't securly distribute the keys, you're fucked.

The wetting layer is a by-product of the method used to grow the dots.I can talk about InAs dots in GaAs, which are the ones I know best. In order to grow the self-assembled dots, you first grow enough layers of GaAs so that you end up with an atomically flat layer of GaAs. Then, you start growing layers (one atomic layer at a time, such is the magic of molecular beam epitaxy!), until a certain "critical height" (I think it's around 5 monolayers). At that time, you stop the growth for a little while and the InAs layer spontaneously forms "droplets" in the GaAs surface, which will be the quantum dots after you grow some GaAs on top. The problem is that the droplets don't use up all the InAs that was deposited, so some remains in the surface and forms the so-called "wetting layer" which behaves similarly to a quantum well. I don't remember any references off the top of my head, but look for articles on the "Stranski-Krastanov growth mode".